Facile fabrication of Eu-based metal–organic frameworks for highly efficient capture of tetracycline hydrochloride from aqueous solutions

The tetracycline hydrochloride (TCH) removal from wastewater is important for the environment and human health yet challenging. Herein, the Eu-based MOF, Eu(BTC) (BTC represents 1,3,5-trimesic acid) was prepared by an efficient and environmental-friendly strategy, and then was used for the TCH capture for the first time. The Eu(BTC) was characterized by different methods such as X-ray diffraction, scanning electron microscopy and Fourier-transform infrared spectroscopy. The TCH uptake of Eu(BTC) was investigated systematically. The influences of experiment conditions such as solution pH value, adsorption time and initial concentration on TCH capacity of Eu(BTC) were also studied. The Eu(BTC) obtained exhibited remarkable TCH uptake (qm was up to 397.65 mg/g), which was much higher than those of most materials such as UiO-66/PDA/BC (184.30 mg/g), PDA-NFsM (161.30 mg/g) and many carbon-based materials reported till now. Besides, the TCH adsorption behavior on Eu(BTC) was explored by Freundlich and Langmuir equations, and the adsorption mechanism was further analyzed. The experimental results suggested that the TCH adsorption mechanism of Eu(BTC) included the π–π interaction, electrostatic interaction and coordinate bonds. The excellent TCH adsorption performance and the efficient fabrication strategy make the Eu(BTC) prepared promising in TCH removal.

www.nature.com/scientificreports/ Among the adsorbents mentioned above, MOFs are composed of divergent metal ions or clusters and organic ligands [29][30][31][32] , and demonstrate unique properties such as high surface area, tunable pore size, and tailorable functionalities, which are unmatched by conventional materials. However, most MOFs often have low stability in aqueous solution, and their applications are usually focused on the adsorption of organic molecules, drug delivery vectors, luminescence and catalysis [33][34][35] . There have been few reports about the antibiotic removal using lanthanide MOFs to date 36 . In this work, an eco-friendly MOF, Eu(BTC) (BTC means 1,3,5-trimesic acid), was prepared by a facile strategy, and employed for the TCH removal from aqueous solution for the first time. The TCH adsorption kinetics and adsorption isotherms of Eu(BTC) were examined in details. Besides, the influence of pH value in solution, adsorption time and beginning concentration on the TCH adsorption performance of Eu(BTC) were also fully studied. The adsorption data were fitted with Freundlich and Langmuir equations to investigate the behavior of TCH on Eu(BTC). The experimental results indicated that the TCH adsorption capacity of Eu(BTC) was mainly depended on the synergetic effect of the π-π interaction and chemisorption. The reusability and stability of Eu(BTC) in water were studied according to the literatures reported [37][38][39][40] . The Eu(BTC) prepared may offer a promising alternative for the antibiotic removal from waste water. , v/v) in advance, and then the reaction precursor and blended solution were put into a round-bottomed flask, heated and accompanied with continuous stirring at 80 °C for 24 h. When the reaction finished, the flask was cooled to room temperature. The resulting residue was washed continuously with anhydrous ethanol and deionized water until the colorless product was observed, and then the sample was dried at 60 °C overnight to obtain the target Eu(BTC).

Characterization of Eu(BTC).
The specific surface area, pore diameter determination and pore volume were calculated by the BET methods (Quantochrome NOVA, USA). The powder X-ray diffraction pattern of the Eu(BTC) was acquired by powder X-ray diffractometer (Bruker D8, Germany). The FT-IR spectra of Eu(BTC) was collected based on a FT-IR spectrometer in the 400-4000 cm −1 range (NICOLET 380, USA). elemental composition of the samples was characterized by X-ray photoelectron spectroscopy (ESCALAB 250Xi spectrometer, USA). Scanning electron microscopy was applied to observe the morphologies and structures of the products (FEI quanta 400feg, USA). The optical property of samples was investigated using an ultraviolet-visible spectrophotometer (UV-Vis, TU-1901, USA). Adsorption experiments. The adsorption capacity of Eu(BTC) was performed by batch sorption experiments. The influence of experimental conditions such as solution pH (2-10), adsorption time (2-30 h), temperature (298-328 K) and TCH initial concentration (40-140 mg/g) on the adsorption performance was evaluated, respectively. In a typical adsorption procedure, the TCH was dissolved in ultrapure water to obtain the stock solutions (200 mg/L). Standard solutions (20-140 mg/L TCH) were prepared by diluting the stock solutions. All the adsorption time was 24 h, ensuring the establishment of the adsorption equilibrium. The residual amount of TCH in the solution was analyzed by UV-Vis absorption spectroscopy at 357 nm.
Adsorption isotherms. 5 mg Eu(BTC) was placed into 10 mL solution with different TCH concentrations (20,40,60, 80, 100, 120 and 140 mg/L), respectively. After 24 h, at 298, 303, 313, 323 and 333 K, 5 mL solution of each sample was periodically taken out for analysing the TCH concentration remained. The calibration curve was acquired according to the spectrum of the standard TCH solutions. The TCH adsorption capacity of Eu(BTC) was calculated using the Eq. (1).
where q e (mg/g) means the TCH amount adsorbed when reaching the adsorption equilibrium, C 0 (mg/L) is the TCH initial concentration and C e (mg/L) is the equilibrium concentration, m (g) means the dosage of Eu(BTC) and V (L) represents the TCH solution volume used.
Adsorption kinetics. The adsorption kinetics was investigated at the predetermined time interval from 2 to 30 h. 50 mg Eu(BTC) was dispersed into 100 mL TCH solution (100 mg/L) at 298 K. 5 mL solution was sampled at definite time interval and then was analyzed by spectrophotometer.
Influences of pH. 5 mg Eu(BTC) was immersed into different TCH solutions (10 mL, 100 mg/L) with pH from 2 to 10. The pH in aqueous solution was adjusted with dilute HCl or NaOH solution. After reaching the adsorption equilibrium, the residual TCH concentration was determined by UV spectrophotometer, respectively.

Results and discussion
Characterization. BET analysis. For examining the porosity of Eu(BTC), the nitrogen adsorption/desorption isotherms were measured. The nitrogen adsorption-desorption curve and pore size distribution curve of Eu(BTC) before and after TCH adsorption were shown in Fig. 1. It can be seen that the Eu(BTC) displayed a good nitrogen adsorption ability at the relative pressure (P/P 0 ) from 0 to 1, and the nitrogen isotherms was consistent with the representative type IV isotherm, which suggested the existence of mesopores in Eu(BTC). As shown in Table 1, the calculated surface area (S BET ) of Eu(BTC) was 123.87 m 2 /g and the average pore diameter was 8.81 nm. Compared with the Eu(BTC), the S BET and pore volume of Eu(BTC)-TCH (after TCH adsorption) significantly decreased, which were attributed to the pore filling effect in the process of TCH adsorption 41 .
XRD analysis. The XRD measurement was carried out to investigate the crystalline structure of Eu(BTC) before and after TCH adsorption. The characteristic diffraction peaks of Eu(BTC) were displayed in Fig. 2. The diffraction pattern exhibited two distinct peaks at 8.54° and 10.50°, which was agreed with the simulated patterns reported 42 . The diffraction peaks were narrow and strong, indicating the excellent crystallinity of Eu(BTC) obtained. In addition, there was almost no impurity peaks appeared in the diffraction pattern, suggesting the ideal purity and crystallinity of the Eu(BTC). The position of diffraction peaks remained unchanged before and after TCH adsorption, conforming the perfect stability of Eu(BTC).
FT-IR analysis. The FT-IR spectra of Eu(BTC) before and after the TCH adsorption, H 3 BTC and TCH were measured. As shown in Fig. 3, H 3 BTC displayed three characteristic peaks at 3086 cm −1 (the stretching vibration peak of -OH), 1712 cm −1 (the stretching vibration peak of -C=O) and 525 cm −1 (the bending vibration peak of -C=O). The above three characteristic peaks disappeared in the spectrum of Eu(BTC), indicating that the BTC ligands were completely deprotonated after the reaction. The peaks of Eu(BTC) at 1543-1651 and 1373-1388 cm −1 can be allocated to the stretching vibrations and the bending vibrations of C=O, respectively. The broad band at 3394 cm −1 was assigned to the hydrogen-bonded vOH groups from the water adsorbed, suggesting that the water molecules were successfully coordinated with Eu 3+ . These results demonstrated the successful synthesis of Eu(BTC). In the case of TCH, the peaks at 1589, 1620 and 1666 cm −1 could be assigned to the stretching vibrations of -C=O in ring C, ring A and amide I (the C=O group of the -CONH 2 ), respectively. Compared with that of Eu(BTC), the main absorption peaks of Eu(BTC)-TCH kept almost unchanged, while the wavenumber migration occurred to some extent. For the Eu(BTC)-TCH, the peak at 1620 cm −1 could be attributed to TCH, meaning the successful TCH adsorption of Eu(BTC). The peaks of Eu(BTC) at 3394, 1651 and 818 cm −1 were all significantly weakened, confirming the TCH absorption of Eu(BTC) again. The strong peak at 818 cm −1 was  XPS analysis. The X-ray photoelectron spectroscopy (XPS) analysis was performed to investigate the composition and chemical status of Eu(BTC). The full spectrum of Eu(BTC) and Eu(BTC)-TCH displayed that the primary chemical components were C, O and Eu (Fig. 4a,b). As shown in the detailed spectra of Eu 3d (Fig. 4c,d), the peak at 1165.2 eV could be corresponded to the Eu 3d 3/2 , the peak at 1135.5 eV was assigned to the Eu 3d 5/2 of Eu(BTC), and the peak at 1135.4 eV was ascribed to the Eu 3d 5/2 of Eu(BTC)-TCH 43 . The C 1s spectrum (Fig. 4e,f) of Eu(BTC) before and after TCH adsorption exhibited four peaks centered at 289.65-290.15 (π-π*), 288.55-288.75 (C=O), 286.50-286.8 (C-O) and 283.4-238.5 eV (C-C). All the C 1s peaks shifted to higher binding energy after the TCH adsorption, indicating that these functional groups might improve the TCH adsorption.
SEM analysis. The structure and morphology of Eu(BTC) were observed by scanning electron microscopy (SEM). As displayed in Fig. 5, the overall appearance of Eu(BTC) was rod-shaped, and the particles were reunited and gathered into clusters. Part of the boundaries between particles disappeared and fused into larger particles, and irregular deformation appeared for some particles.   Table 2) 44 . The anionic content of TCH will increases with the rise of pH value, which benefits the TCH adsorption because of the strong electrostatic attractions 45 and the cations of the adsorbent in solution will reduces with the increasing pH 46 . As shown in Fig. 6c and d, the TCH adsorption amount of Eu(BTC) increased quickly when the pH value changed from 2 to 6, and then leveled www.nature.com/scientificreports/ off at pH 6 ~ 8. The TCH uptake decreased rapidly when the pH value rose further. For investigating the TCH adsorption mechanism, the zeta potential changes of Eu(BTC) were measured and illustrated in Fig. 7. When the pH value was lower than 3.3, the repulsive interaction caused by the positively charged TCH + and the positive groups of Eu(BTC), afforded a poor TCH adsorption. At about pH 3.3 ~ 7.7, the TCH remained the zwitterions form, and its adsorption amount did not decline with the pH increase, indicating that the repulsive interaction was not the main factor in the adsorption process. At pH 6, the Eu(BTC) demonstrated the largest adsorption capacity, which can be attributed to the π-π interactions of benzene rings and coordinate bonds. In addition, When the pH value was higher than 7.7, the negative charge of TCH (TCH − and TCH 2-) and OH − increased gradually, the Eu(BTC) contained abundant anions, thus, the strong repulsive force between the TCH and the Eu(BTC) caused a low TCH adsorption. While the adsorption capacity decreased, plenty of TCH adsorbed remained, possibly attributing to the pore filling.
Effect of ionic strength. The effect of ionic strength on TCH adsorption of Eu(BTC) was examined. As displayed in Fig. S1, Na + did not significantly affect the TCH adsorption of Eu(BTC) (M ≤ 0.02). However, with the increasing of Na + concentration from 0.06 to 0.08 mol/L, the TCH adsorption amount decreased, which could be ascribed to the competitive adsorption between the Na + and TCH on Eu(BTC) via electrostatic interaction.  Fig. 8. To explore the adsorption process, all the experimental results were matched with the pseudo-first-order (Fig. 9a,c,e) and the pseudo-second-order kinetic (Fig. 9b,d,f) and the fitting parameters obtained were enumerated in Table 3. The fitting parameters was calculated according to the following equation: The pseudo-first-order equation: The pseudo-second-order equation: where q t (mg/g) represents the TCH amount adsorbed of Eu(BTC) at time t, k 1 (h −1 ) is the constant of pseudofirst-order model and k 2 (g/mg min) means the equilibrium pseudo-second-order rate constant.
(2) ln q e − q t = ln q e(exp) − K 1 t  In the initial stage, the adsorption rate increased with the increasing adsorption time, and then slowed down as the time proceeded. The fast TCH adsorption rate in the beginning was ascribed to the abundant active sites in the unoccupied surface of Eu(BTC). The laboratory data were fitted with both the pseudo-first-order model and pseudo-second-order model (0.90≦R 2 ≦0.99). The R 2 values of the pseudo-second-order model were in the range of 0.9864 ~ 0.9926, which were higher than that of the pseudo-first-order model (0.9857 ~ 0.9925),  Effect of initial TCH concentration: adsorption isotherms. The adsorption isotherm was of great importance in assessing the maximum adsorption capacity and providing insight into the adsorption mechanism. To investigate the interaction between adsorbate and adsorbent, the adsorption equilibrium data of Eu(BTC) at different initial concentrations (20 ~ 140 mg/L) were simulated with two usually used isotherm models, Langmuir and Freundlich models. The TCH adsorption isotherms of Eu(BTC) were studied at 298, 303, 313, 323 and 333 K. The Langmuir, Freundlich and Dubinin-Radushkevich isotherm models were expressed as Eqs. (4), (5) and (6), respectively. Langmuir: Freundlich: Dubinin-Radushkevich: where q m (mg/g) represents the theoretical maximum adsorption amount, k L refers to the constant of Langmuir equilibrium adsorption, k F is empirical constant that represents the Freundlich constants, n is the no-linearity constant 48,49 , respectively. The parameters of these adsorption isotherms were listed in Table 4 in details. β (mol 2 / KJ 2 ) is the D-R constant, ɛ (KJ 2 /mol 2 ) is the polanyil potential, and q m is the adsorption capacity.
With the initial TCH concentration increased, the TCH equilibrium adsorption capacity of Eu(BTC) gradually increased, and then tended to balance (Fig. 10). Meanwhile, it was not hard to find that the temperature has a remarkable influence on TCH adsorption capacity, and high temperature benefits the TCH uptake. The q e -C e curve ascended as the temperature increased, confirming that the TCH adsorption of Eu(BTC) was a distinct endothermic process. The theoretical maximum adsorption amount calculated by the Langmuir isotherm model was up to 397.65 mg/g at 303 K, which was higher than those of many adsorbents reported (Table S1). Compared with the results computed by Langmuir model, the Freundlich model provided a larger R 2 value, suggesting that the multi-layer adsorption existed in the TCH adsorption of Eu(BTC). The results of Dubinin-Radushkevich model were presented in Fig. S2, and the corresponding parameters obtained were listed in Table S2. When the E value is between 8 and 16 kJ/mol, the sorption mechanism is assigned to the ion exchange process, and in the case of lower E value (less than 8 kJ/mol), it is the physical sorption 50 . The E values calculated in this study were 0.1066 to 0.2259, suggesting that the TCH adsorption processes of Eu(BTC) was controlled by physical adsorption.
Effect of temperature: adsorption thermodynamics. In order to examine the temperature effect on TCH adsorption of Eu(BTC), the isothermal adsorption experiments were carried out at different temperatures and standard entropy (ΔS°). The thermodynamic parameters were calculated according to the following Eqs. (7) and (8): www.nature.com/scientificreports/ where K d means the distribution coefficient, ΔS° (J/mol·K) represents the entropy variation, R (8.314 J/mol/K) means the universal constant of ideal gases, ΔH° (kJ/mol) is the enthalpy change, T (K) represents the absolute temperature, and ΔG° (kJ/mol) is the Gibbs free energy variation 51 , respectively. The thermodynamic parameters obtained were summarized in Table S3. As displayed in Fig. 8, the TCH adsorption capacity of Eu(BTC) continuously increased when the temperature rose. The ΔG of the system was less than zero (− 6.0229 to − 0.4070 kJ/mol), indicating that the TCH adsorption process was spontaneous. The ΔH was bigger than zero (0.1413 kJ/mol), suggesting that the TCH adsorption of Eu(BTC) was endothermic, that is, increasing the temperature could improve the adsorption performance 19 . The ΔS was 0.001840 kJ/mol/K, demonstrating the excellent binding affinity between the Eu(BTC) and TCH, and the increased disorder at the solid-liquid interface during the TCH adsorption process.

Reusability of Eu(BTC).
The reusability is a significant factor in evaluating the practical application of adsorbents. The recyclability of Eu(BTC) was examined, as can be seen in Fig. S3. After saturated with TCH, the adsorbent was desorbed and then re-employed for TCH capture. The TCH adsorption was conducted for four times, and then no obvious decline of uptake (less than 2.87%) was observed, indicating the excellent reusability of Eu(BTC). As displayed in Fig. S4, the positions of Eu(BTC) diffraction peaks remained constant after four adsorption and desorption cycles, confirming the outstanding stability of Eu(BTC). It makes the Eu(BTC) Table 3. Nonlinear fitting kinetic parameters of TCH adsorption on Eu(BTC).

T (K)
Pseudo-first-order kinetic model  www.nature.com/scientificreports/ obtained highly promising in practical application such as the TCH removal from aqueous solutions for environmental remediation.

Conclusions
In this work, we reported an efficient and environmental-friendly method for the synthesis of Eu(BTC). The pseudo-second-order adsorption model was used to investigate the interaction between TCH and Eu(BTC), and the existence of chemical adsorption in the process of TCH capture was confirmed. The fitting curves of the Freundlich isotherm model exhibited better linearity than that of Langmuir isotherm model, indicating that the TCH adsorption of Eu(BTC) was a heterogeneous process. The maximum TCH uptake of Eu(BTC) was up to 397.65 mg/g, which was much higher than most benchmark adsorbents reported. The negative value of ΔG and the positive value of ΔH revealed the spontaneous and endothermic nature of the TCH adsorption process on Eu(BTC). The experimental facts demonstrated that the TCH adsorption mechanism of Eu(BTC) was the coexistence of π-π* interaction and chemisorption. The excellent TCH adsorption capacity and reusability of Eu(BTC) sheds a new light on antibiotics removal from wastewater.

Data availability
All data generated or analyzed during this study are included in this published article and its supplementary information file.